So the original equation would look like: CO(g)+O2(g) --> CO2(g)
We can count the number of elements on each side of the equation to determine if it is balanced.
For A CO(g)+O2(g) --> CO2(g) there are 3O in the product side an 2O in the reactant side, so it is not balanced
For B CO(g)+2O2(g) -->2CO2(g) there are 2C on the product side, but only 1 C on the reactant side so it is not balanced.
For C 2CO(g)+O2(g) --> 2CO2(g) there are 2C on the product side and 2C on the product side, so far it is correct, we'll them check the O.
There are 4O on the reactant side and 4O on the product side. So it is correctly balanced.
Meaning your answer is
C.) 2CO(g)+O2(g) --> 2CO2(g)
